A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone. The elastic properties of tendons modulate forces during locomotion, providing additional stability with no active work. They also store and recover energy at high efficiency. Normal healthy tendons are composed primarily of parallel arrays of type I collagen fibers closely packed together, but also include a small amount of elastin and of proteoglycans. Due to their highly specialized ultrastructure, low level of vascularization and slow collagen turnover, tendons are very slow to heal if injured, and rarely regain their original strength. Partial tears heal by the rapid production of disorganized type-III collagen, which is weaker than normal tendon. Recurrence of injury in the damaged region of tendon is common.
Tendinopathies are chronic disorders or injuries of the tendons, that appear to result from an imbalance between catabolic and anabolic responses that result from gradual wear and tear to the tendon from overuse or aging. The result of this imbalance is tendon degeneration, weakness, tearing, and pain. In contrast, acute tendon injuries such as, for example, tendon rupture or detachment from the bone are quite sudden and usually require surgery to repair the rupture or reattach the tendon to bone. Anyone can develop a tendinopathy, but people who tend to make the same motions over and over again in their jobs, sports, or regular daily activities are more likely to develop them. Tendinopathy usually causes pain, stiffness, and loss of strength in the affected area.
The term tendinopathy refers to two types of tendon injury: tendinosis and tendinitis. The term also encompasses tenosynovitis, a tendinopathy of the outer lining of the tendon which occurs in certain tendons such as flexor tendons and the Achilles tendon.
Tendinosis is a non-inflammatory injury to the tendon characterized by intratendinous degeneration of the tendon usually in the form of microtears in the tissue in and around the tendon caused by overuse, leading to an increase in the number of tendon repair cells around the area of damage. Degeneration of the tendon is caused by damage to or disorganization of the collagen fibers, cells, and vascular components of the tendon, which can reduce the tendon's tensile strength and can lead to tendon rupture if not treated. The changes in collagen organization are characterized by separation/loosening/crimping of fibers, loss of parallel orientation, decrease in fiber diameter and decrease in overall density of collagen. In addition, collagen microtears can also occur that are surrounded by erythrocytes, fibrin, and fibronectin deposits. On the other hand, there is an increase in type III (reparative) collagen. These matrix organization changes can lead to decreased birefringence under polarized light microscopy. In addition to collagen content and organization, tendinosis is also characterized by an increase in mucoid ground substance (proteoglycans) and variation in cellular density in affected areas. Some areas contain abnormally plentiful tenocytes, with rounded nuclei and ultrastructural evidence of increased production of proteoglycan and protein. In contrast, other areas of the affected tendon may contain fewer tenocytes than normal, with small, pyknotic nuclei. Another characteristic feature of tendinosis is proliferation of capillaries and arterioles. Several subcategories of tendon degeneration in tendinosis have been identified by electron microscopy: (1) hypoxic degeneration, (2) hyaline degeneration, (3) mucoid or myxoid degeneration, (4) fibrinoid degeneration, (5) lipoid degeneration, (6) calcification, and (7) fibrocartilaginous and bony metaplasia. These pathologies can coexist with varying prevalence, depending on the anatomical site and the nature of the insult that caused them (e.g., hypoxia versus mechanical loading; acute versus chronic injury). For example, mucoid degeneration area is characterized by light microscopy, large mucoid patches and vacuoles between fibers. However, lipoid degeneration is characterized by abnormal intratendinous accumulation of lipid that results in disruption of collagen fiber structure. In some cases, tendinosis is accompanied by focal necrosis or calcification of the tendon. It is a very common reason for chronic pain surrounding a joint. Tendinosis is also characterized by an absence of the initial inflammatory response. Inflammatory cells are thought to be early stage mediators of the repair process, without which tendinosis can become a chronic condition.
Characteristic increases in water content and disorganization of the collagen matrix associated with tendinosis can be diagnosed by ultrasonography or magnetic resonance imaging. Symptoms can vary from simple aching and stiffness in the local area of the tendon to a burning sensation surrounding the entire joint around the injured tendon. For many patients, the pain is frequently worse during and after activity, and the tendon and joint area can become stiffer the following day as swelling impinges on the movement of the tendon.
Tendinitis is an inflammatory injury to the tendon, characterized by degeneration like that observed in tendinosis, but also accompanied by inflammation of the tendon accompanied by vascular disruption and an inflammatory repair response. Tendinitis is often accompanied by fibroblastic and myofibroblastic proliferation, as well as hemorrhage and organizing granulation tissue. Generally tendinitis is referred to by the body part involved, such as Achilles tendinitis (affecting the Achilles tendon), or patellar tendinitis (also known as “jumper's knee,” affecting the patellar tendon), though there are certain exceptions, such as lateral epicondylitis (also known as “tennis elbow,” affecting the Extensor Carpi Radialis Brevis tendon). Symptoms can vary from aches or pains and local stiffness to a burning sensation surrounding the entire joint around the inflamed tendon. In some cases, tendonitis is characterized by swelling, sometimes accompanied by heat and redness; there may also be visible knots surrounding the joint. For many patients, the pain is usually worse during and after activity, and the tendon and joint area can become stiffer the following day as muscles tighten from the movement of the tendon.
Current treatments are primarily palliative in nature, with treatment traditionally focusing on anti-inflammatory measures, including treatment with nonsteroidal anti-inflammatory drugs (NSAIDs), steroid injections, and physical therapy, despite the fact that tendinosis tends not to be associated with an inflammatory response. More recently, shock wave therapy, low-level laser therapy, sclerotherapy, and other experimental treatments have been tested. For the most part, it appears that some treatments (e.g., NSAIDs and cortisone injections) offer short-term relief, while the longer-term benefit of current treatments remains unclear. Therefore, there is a need for improved methods of treating tendinopathies that offer longer-term benefits compared to existing treatment modalities.
PDGF is stored in the alpha-granules of platelets and is secreted during tissue repair by locally-activated cells, including macrophages, fibroblasts, and endothelial cells. PDGF-BB is one of the major products of the hemorrhage and inflammation of acute tendon injury. Platelet-derived growth factor-BB (PDGF-BB) is a wound healing protein which is known to be chemotactic (cell migration) and mitogenic (cell proliferation) for cells of mesenchymal origin, including bone (osteoblast) and tendon (tenocyte) cells. Additionally, PDGF-BB has been shown to up-regulate vascular endothelial growth factor (VEGF), leading to increased angiogenesis (revascularization), which is essential for successful regenerative processes.
The Achilles tendon is the thickest and strongest tendon in the human body, which allows it to support high loads. The mechanical loading environment in which the Achilles tendon functions makes it prone to rupture. Achilles tendon ruptures can occur as a result of a variety of factors, however rupture is often associated with degenerative changes. (Mafulli N, Wong J, Almekinders L. Types and epidemiology of tendinopathy. Clinics in Sports Medicine. 2003; 22:675-692). Following the repair process, ruptured Achilles tendons demonstrate a reduction in type I collagen and a relative increase in the amount of type III collagen. This change in composition leads to less cross-linking and reduced tensile strength. Even after healing, a ruptured Achilles tendon remains weaker due to hypercellularity, disorganization, and decreased collagen cross-linking (Maffulli N, Moller H D, Evans C H. Tendon Healing: Can it be Optimized? British Journal of Sports Medicine, 2002; 36:315-316). Controversy exists regarding the optimal treatment for Achilles tendon ruptures, with pros and cons to both conservative (non-operative) and surgical therapies. Non-operative treatment results in a higher re-rupture rate and decreased strength but avoids the costs and risks associated with surgery. (Inglis A E, Scott W N, Sculco T P, et al. Ruptures of the tendo achillis: an objective assessment of surgical and non-surgical treatment. J Bone Joint Surg Am. 1976; 58:990-993; Nistor L. Surgical and Nonsurgical treatment of Achilles tendon rupture: a prospective randomized trial. J Bone Joint Surg Am 1981 63(3):394-9; Chalmers J. Review Article: Treatment of Achilles tendon ruptures. Journal of Orthopaedic Surgery 200 8(1):97-99). Surgical repair carries with it the risks of surgery and anesthesia; however it provides increased strength, lower re-rupture rates and a earlier return to athletic activities. (Nistor L. Surgical and Nonsurgical treatment of Achilles tendon rupture: a prospective randomized trial. J Bone Joint Surg Am 1981 63(3):394-9; Rettig A, Liotta F J, Klootwyk T E, Porter D A, Mieling P. Potential Risk of Rerupture in Primary Achilles Tendon Repair in Athletes Younger than 30 years of Age. Am J of Sports Med 2005: 33(1):119-123) Regardless of a clinician's preference for treatment of acute Achilles tendon ruptures, surgical repair will continue to have its place in the spectrum of treatment of these injuries in the active patient population. Augmentation of the biological repair process, thereby improving tendon healing, could potentially lead to a faster return to activity and improved clinical outcomes compared to current treatment modalities.
There have been several in vivo and in vitro studies regarding biologic augmentation of tendon healing. See e.g.: Seeherman H J, Archambault J M, Rodeo S A, et al. rhBMP-12 accelerates healing of rotator cuff repairs in a sheep model. J Bone Joint Surg Am. 2008; 90(10):2206-2219; Chan B P, Fu S C, Qin L, et al. Supplementation-time dependence of growth factors in promoting tendon healing. Clin Orthop Relat Res. 2006; 448:240-247; Uggen J C, Dines J, Uggen C W, et al. Tendon gene therapy modulates the local repair enviroment in the shoulder. J Am Osteopath Assoc. 2005; 105(1):20-21; Gelberman R, Thomopoulos S, S akiyama-Elbert S, et al. The early effects of sustained platelet-derived growth factor administration on the functional and structural properties of repaired intrasynovial flexor tendons: an in vivo biomechanic study at 3 weeks in canines. J Hand Surg Am. 2007; 32(3):373-379; Thomopoulos S, Das R, Silva M J, et al. Enhanced flexor tendon healing through controlled delivery of PDGF-BB. J Orthop Res. 2009; 27(9):1209-1215; Thomopoulos S, Zaegel M, Das R, et al. PDGF-BB released in tendon repair using a novel delivery system promotes cell proliferation and collagen remodeling. J Orthop Res. 2007; 25(10):1358-1368; Dines J, Grande D, Dines D. Tissue Engineering and Rotator Cuff Tendon Healing. J Shoulder Elbow Surg, September/October 2007: 204S-206S.
Delivering rhPDGF-BB to the site of repair in sufficient doses and over the proper time-course is important in achieving the desired clinical effect. Several studies describe sutures coated with biologics. See e.g. Rickert M, Jung M, Adiyaman M, Richter W, Wimank H G. Growth and differentiation factor 5 coated suture stimulates tendon healing in an Achilles tendon model in rats. Growth Factors 2001; 19:115-126; Weiler A, Forster C, Hunt P, Falk R, Jung T, Unterhauser F N, Bergmann V, Schmidmaier G, Haas N P. The Influence of Locally Applied Platelet-Derived Growth Factor-BB on Free Tendon Graft Remodeling After Anterior Cruciate Ligament Reconstruction. American Journal of Sports Medicine 2004; 32(4):881-891; Dines J, Weber L, Razzano P, et al. The Effect of Growth Differentiation Factor-5-Coated Sutures on Tendon Repair in a Rat Model. J Shoulder Elbow Surg 2007; 16:2155-2215; Uggen C, Dines J, McGarry M, et al. The effect of Recombinant Human Platelet Derived growth Factor BB coated sutures on Rotator cuff Healing in a Sheep Model. Arthroscopy: 2010: 26(11): 1456-1462.
What is needed are improved sutures for delivery of PDGF to a tendon, for example, for repair of ruptured tendon such as ruptured Achilles tendons.